The introduction of LTE (Long Term Evolution) has been considered as a next-generation mobile communication system, and is being standardized. In the LTE, it is possible to improve the communication speed compared to current third-generation mobile communication systems (e.g., W-CDMA (Wideband Code Division Multiple Access)).
Specifically, in the LTE, OFDMA (Orthogonal Frequency Division Multiple Access) is employed for a downlink radio access system, and SC (Single Carrier)-FDMA is employed for an uplink radio access system.
The OFDMA is a digital modulation system for multiplexing a plurality of carrier waves (typically referred to as subcarriers) using the orthogonality of frequencies. One of the characteristics of the OFDMA is its high tolerance against fading and multi-path interference.
In contrast, the SC-FDMA has characteristics that are similar to those of the OFDMA. The SC-FDMA is different from the OFDMA in that it continuously allocates the subcarriers to users. Thus, the SC-FDMA is expected to improve power efficiency in comparison with the OFDMA. The uplink radio resources are divided into frequency components and time components, and the divided radio resources are allocated to users.
Further, various measures to make an effective use of limited radio resources in the LTE have been examined for the purpose of improving the communication speed in the whole system. One of these measures includes a function of distributing load among cells. Roughly speaking, this function is to cause UE (User Equipment) to be handed over to a neighboring cell when there are mobile stations (UE) concentrated in one cell, to avoid a situation in which load is applied only on specific cells. Accordingly, it is expected that both of the throughput of UE that exists in the cell boundary and the throughput of the whole system are improved.
Specifically, a handover in the LTE is executed when triggered by Measurement Report from UE to a radio base station (eNB: enhanced Node B). Events A1-A5 are defined in NPTL 1 as a type of the Measurement Report. Among Events A1-A5, Event A3, which specifically triggers the handover, is transmitted from the UE when the condition shown in the following equation (1) is satisfied.Mn+Ofn+Ocn−Hys>Ms+Ofs+Ocs+Off  (1)
In the left-hand side of the above equation (1), Mn denotes reception quality in a neighboring cell. The symbol Ofn is an offset regarding the frequency band used in the neighboring cell. The symbol Ocn is an offset specific to the neighboring cell. The symbol Hys is a hysteresis parameter regarding Event A3. Meanwhile, in the right-hand side of the above equation (1), Ms denotes reception quality in a serving cell. The symbol Ofs is an offset regarding the frequency band used in the serving cell. The symbol Ocs is an offset specific to the serving cell. The symbol Off is an offset parameter regarding Event A3.
As a rule, the UE transmits Event A3 to the eNB when the reception quality Mn in the neighboring cell measured by itself exceeds the reception quality Ms in the serving cell, to thereby request the handover. Note that the reception qualities Mn and Ms can be included in Event A3.
Incidentally, when the value of the offset Ocn becomes larger, it is easier to satisfy the condition shown in the above equation (1). In contrast, when the value of the offset Ocn becomes smaller, it is more difficult to satisfy the condition shown in the above equation (1). In short, by changing the offset Ocn, it is possible to make it easy to perform handover, or to make it difficult to perform handover of the UE to a specific neighboring cell.
Thus, when the cell load of the eNB is high, the eNB changes the offset Ocn, includes the change in broadcast information or a control signal to notify it to the UE, thereby causing the UE to transmit Event A3. Accordingly, the handover by the load distribution function is executed.
Further, the eNB typically uses PF (Proportional Fair) scheduling when allocating radio resources to UE. In the PF scheduling, the fairness of the throughput for each cell and the throughput among UE is considered. In other words, when there are numerous UE in the cell of the eNB, the eNB may not be able to guarantee the service quality required by each UE.
Accordingly, the eNB which is a handover destination of the UE (hereinafter sometimes referred to as a handover destination eNB) estimates the cell load of itself and determines whether the eNB is able to accept a new call (more specifically, bearer) according to the following equation (2). When the condition shown in the following equation (2) is satisfied, the handover destination eNB accepts the call. In the following description, the eNB communicated with the UE is denoted by a handover source eNB to differentiate it from the handover destination eNB.L—c+L—d<L—th  (2)
In the above equation (2), L_c denotes a current load amount in the handover destination eNB. Further, L_th denotes a threshold to determine whether it is possible to accept a call (hereinafter referred to as a call admission threshold). Furthermore, L_d denotes a load amount that is increased as a result of accepting the new call (hereinafter referred to as a load increase amount), and a fixed value is typically used as the load amount. The reason why the fixed value is used is that since the handover destination eNB does not have information regarding the radio status of UE, it is impossible to accurately estimate the load increase amount L_d.
As a reference technique, PTL 1 discloses a mobile communication system in that an upper node connected to a handover source eNB and a handover destination eNB starts transmission of user data to the handover destination eNB in parallel with the call admission control stated above when the service quality required by UE is high, thereby mitigating the load according to the processing of transferring user data between both eNBs.